In six brief chapters, mixed-age grouping of young children in schools and child care centers is explored and advocated. Chapter 1 defines mixed-age grouping, examines limitations of single-age grouping, and points out positive characteristics of mixed-age classes. Chapter 2 discusses social development as seen in children's interactions in mixed-age groups. Various studies are cited that focus on how children perceive one another and adapt their behavior and expectations accordingly, how children exhibit specific prosocial behaviors in mixed-age situations, and how children's group participation varies. Chapter 3 reviews studies on the cognitive effects of mixed-age grouping, concluding that psychologists and educators do not yet fully understand how mixed-age interaction affects cognitive development, and calling for more research on the interactive processes involved and the teacher's role in them. Chapter 4 discusses two strategies for mixed-age learning: peer tutoring and cooperative learning. Chapter 5 describes examples of successful implementation of mixed-age programs, including a 2-year kindergarten at the University of Northern Iowa's Malcolm Price Laboratory School, and the Fajans School in Sweden where elementary school-age children were not grouped by age or ability. Chapter 6 addresses four basic questions about implementing mixed-age grouping. A brief section giving conclusions and recommendations is provided, followed by 63 bibliographic citations and appended suggestions for teachers working with mixed-age groups. (RH)